But dig deeper into the research and you’ll see an important line: “The data used in this study are based on display ads in browsers (desktop and mobile) and does not include mobile in-app or video apps.”

For a company that uses the plural of ‘data’ properly, this is a telling omission. Mobile is less open to non-viewable impressions. And, within mobile, full-screen ads make it even less of an issue. Full-screen video ads take this another step further as they ramp up the engagement factor. Taken together, these could radically affect the value of a mobile impression.

The viewability issue

The Media Rating Council (MRC) definition of display ad viewability (and the definition Google quotes in its research) boils down to this: 50 per cent of its pixels must be in view on the screen for a minimum of one second.

So an ad buried at the bottom of a long web page, or a video ad that fails to start, may not be classed as ‘viewable’. Yet they would still be impressions, reported as such to the advertiser, and billed against the budget with no hope of ROI.

This has concerned advertisers for quite some time. A brief history of viewability: the IAB’s first viewability guidelines date back to 2004. In March 2012 the MRC issued its advisory on viewable impressions, warning against adopting industry-wide metrics because of the complications involved. These included thorny issues such as iframes displaying ads on domains different from the pages in which they were displayed, and decision tags simply not firing. In March 2014 the MRC lifted this advisory, having established its viewable ad impression guidelines, as a supplement to the IAB’s earlier work.

With so many great industry initiatives, one would imagine we’re well beyond Wanamaker’s days of "Half the money I spend I waste, I just don't know which half.” But if Google is right about non-viewable impressions accounting for more than half of all ads served, we’ve still some distance to go.

Or have we?

The mobile opportunity…

Google can make this claim only because it omitted mobile in-app or video ads in its study. This is because mobile has much less of a viewability problem than desktop.

When you play your next ad-supported mobile game, notice how the ads are always present on the screen. Even scrolling through a long news story, chances are that the ad is designed to be visible.

This is because apps are designed specifically to fit onto mobile devices, and is why the MRC concurs with the IAB when it says that “ad impressions served in an in-application environment are currently generally assumed to be viewable”. It’s a hugely significant statement, especially when you consider that according to Nielsen, in the US, throughout Q4 2013, smartphone users spent 89 per cent of their time on their mobile devices using apps, not the mobile web.

If apps on mobiles are generally regarded as viewable, and apps take up much more of mobile users’ time, then there is more viewability on mobile compared to desktop, much more of the time.

… plus the mobile full-screen ads opportunity…

But mobile can go even further, with full-screen ads.

Full-screen ads occupy the entire screen and so, by definition, are viewable (accepting that, as the IAB says, 100 per cent viewability isn't possible). A full-screen ad doesn’t disappear below the fold, or share space with competing ads. It’s right in front of the user and is much more effective at triggering a response.

Larger, more compelling ads can stimulate engagement, and tell much more effective, joined-up brand stories than smaller sizes. This is why full-screen ad campaign data shows up to 10x the clickthrough rates of standard banner ads. And this can only improve as screen sizes grow with models such as the iPhone 6.

… plus the mobile video ad opportunity…

We know that mobile video ads work much more effectively than any other format.

Video is attractive to all players in the market. Advertisers and brands ‘get’ video, especially when it’s full-screen. They can tell the same compelling brand stories that they have been doing for decades with TV and movie advertising.

Users also ‘get’ video, for much the same reason: they see the same high-quality, full-screen formats as they do at home, and know that all they have to do is tap to find out more. No clever rich media dynamics or boring static banners.

Mobile and desktop are not the same. So how does increased confidence in viewability affect valuations of mobile impressions versus desktop?

Well firstly, mobile and mobile aren’t even the same where different publishers are concerned. This is why it’s hard to compare quantitative metrics such as clickthrough rates or cost per mille, and many industry analysts freely admit this.

Let’s take a qualitative approach. For a given campaign, on a specific publisher, across mobile and desktop screens, how much confidence could a brand marketer place in mobile’s ability to deliver results, compared to desktop?

The answer lies in the combination of mobile’s viewability, targeting and formats.

Mobile in-app ads are viewable. The IAB and the MRC agree on this.

The targeting data across all digital advertising is extremely rich, incorporating first-, second- and third-party ad tech data, and this is what feeds the algorithms that find the right inventory for the right campaign.

However mobile takes this further, bringing in context and location data and enabling advertisers to reach audiences wherever they are. Bad weather in your city? Advertise umbrellas. Good weather? Sell ice cream. Poor public transport? Tap here to book a taxi… These are all approaches that simply would not appeal to someone sitting at a desk.

The formats available to mobile also go further than desktop. Mobile video is emerging as a star performer, as larger devices such as the iPhone 6 Plus flood the market. And, as we’ve already seen, full-screen mobile ads take this a huge step further.

So is more viewable more valuable? On top of targeting and formats, 2015 should see digital marketers invest more confidence in mobile. And where confidence goes, money inevitably follows.